1. Field of the Invention
The invention relates generally to a control unit and control method for a variable valve timing mechanism. More specifically, the invention relates to a variable valve timing control that is executed when an internal combustion engine is idling.
2. Description of the Related Art
A variable valve timing (VVT) mechanism that changes the phase (i.e., crank angle), at which an intake valve or an exhaust valve is opened/closed, based on the engine operating state has been used. Such variable valve timing mechanism changes the phase of the intake valve or the exhaust valve by rotating a camshaft, which opens/closes the intake valve or the exhaust valve, relative to, for example, a sprocket. The camshaft is rotated hydraulically or by means of an actuator, for example, an electric motor. It is more difficult to obtain the torque for rotating the camshaft, when the camshaft is rotated by the electric motor than when the camshaft is hydraulically rotated. Therefore, when the camshaft is rotated by the electric motor, usually, the torque generated by the electric motor is transferred to the camshaft via, for example, a link mechanism to rotate the camshaft.
Japanese Patent Application Publication No. 2005-98142 (JP-A-2005-98142) describes a valve timing adjustment device which is a type of the above-described variable valve timing mechanism and which changes the direction, in which a target phase is changed, within a short time. In the valve timing adjustment device described in JP-A-2005-98142, as shown in FIGS. 16 and 18, the change ratio (corresponding to the inclination of the tangent line in the graph in each of FIGS. 16 and 18) of a change amount of the rotational phase of a camshaft relative to a crankshaft (i.e., change amount of the valve phase) with respect to a change amount of the rotational phase of a guide rotational body, which rotates in accordance with the operation of an actuator, is changed based on the phase region.
Usually, when an engine is idling, a target value of a valve phase is set to a predetermined value that is suitable for the idling operation. In contrast, in a variable valve timing mechanism with which the change ratio is variably set based on the valve phase region, there is a possibility that the change ratio at the valve phase suitable for the idling operation may not be appropriate depending on the range, in which the phase is allowed to change and which needs to be ensured to deal with various operating states. In addition, when a target idle speed for the engine is variably set based on the vehicle state, it is necessary to derive an appropriate manner for setting the valve phase in accordance with the variably set target idling speed. In the variable valve timing mechanism described above, it is necessary to derive an appropriate manner for setting the valve phase which is used during the idling operation in order to place the valve phase within a region, in which the change ratio is appropriate, when the engine stops. As described above, in the variable valve timing mechanism with which the change ratio of the change amount of the valve phase with respect to the operation amount of the actuator (speed reduction ratio) changes based on the valve phase, it is necessary to take the relationship between the valve phase and the speed reduction ratio into account in order to execute an appropriate valve phase control during the idling operation.